For example, a solid polymer membrane type fuel battery has a stack of multiple cells each of which consists of a membrane of solid polyelectrolyte held between an anode and a cathode. While hydrogen is supplied as a fuel to the anode, air is supplied as an oxidizer to the cathode. Hydrogen ions generated by the catalytic reaction at the anode move through the film of solid polyelectrolyte to the cathode where an electrochemical reaction between the hydrogen ions and oxygen generates electricity.
Fuel batteries like the aforesaid solid polymer membrane type fuel battery generally discharge the unreacted air (hereinafter referred to as the off-gas)to outside the system. Then, it is necessary to confirm that hydrogen gas is not present in the off-gas.
Therefore, systems to confirm the nonpresence of hydrogen gas in the off-gas by means of a hydrogen sensor installed in the discharge system on the cathode side of the fuel battery are proposed, as in Japanese Patent Publication No. 1994-52662 and Japanese Provisional Patent Publication No. 1994-223850.
A contact combustion type gas sensor may be used as the hydrogen sensor. This contact combustion type gas sensor comprises a sensing element carrying a catalyst and a temperature-compensating element carrying no catalyst. This gas sensor determines the concentration of the specimen gas from the difference in electrical resistance between the sensing and temperature-compensating elements by using heat generated when the specimen gas (that is hydrogen when the sensor is a hydrogen sensor) burns on contact with the catalyst.
In order to maintain ion conductivity of the solid polyelectrolyte membrane, the fuel batteries like the aforesaid solid polymer membrane type fuel battery positively moisturize the reacting gas (such as hydrogen or oxygen) supplied to them and produce, when they generate electricity, water of formation by the electrochemical reaction involved in their power generation. Therefore, the off-gas contains heated water and formation water, as a result of which the hydrogen sensor is exposed to the off-gas containing such waters.
As, however, the sensing elements of hydrogen sensors are often gas sensors that work in a heated state like the contact combustion type gas sensors, the heated water or formation water adhering to them creates local nonuniform temperature distribution on the surface thereof that might, in turn, lead to sensitivity lowering and element breakdown.
To eliminate this problem, provision of a gas permeable water-repelling film and aporous silica sheet at the gas intake of containers holding the sensor is proposed, as in Japanese Provisional Patent Publication No. 2000-187014.
Hot and moist fluids, such as those having a temperature of approximately 90° C. and a relative humidity of approximately 100 percent, flow in the off-gas exhaust pipe of fuel batteries like the solid polymer membrane type fuel battery. Provision of a contact combustion type gas sensor that is used in a heated state in such exhaust pipes increases heat-release in the port where the sensor is installed, as a result of which the temperature near the sensor drops to below zero and waterdroplets are formed in the sensor.
To eliminate this problem, the sensor is held in a cap of porous material and heated by a heater provided therearound or temperature drop is prevented by heat insulating material, as proposed in Japanese Provisional Patent Publication No. 1998-233763. However, this solution requires additional work for ancillary facilities for heating or heat insulation, increases installation cost, and hampers size reduction.
The specimen gas may be taken from the exhaust pipe through a sampling passage and led to the sensor through a dehumidifying means. However, this method requires not only large equipment but also correction with moisture amount after removing the measured value. Besides, it also necessitates complex signal processing.
If moisture in the specimen gas adheres and condenses on the porous cap used in the aforementioned prior art, the condensed water comes into contact with the gas-sensing element, thereby creating local nonuniform temperature distribution on the surface of the element that might lead to element breakdown or sensitivity drop.
If the specimen gas flowing in through the cap is not uniformly led to and brought into contact with the sensing and temperature-compensating elements, an imbalance of the flow rate of the incoming specimen gas directly affects the sensing temperature and, thereby, lowers the sensing accuracy.
Keeping the cap that is exposed to and cooled by the flow of the specimen gas at the desired temperature requires a large amount of heat and, thereby, increases power consumption by the gas sensor.
As such, the object of this invention is to provide gas sensors that prevent sensitivity lowering and element breakdown by surely preventing the inflow of the moisture contained in the gas flowing through the fluid path, the wetting of the sensing element and the formation of condensed water in the gas-sensing chamber.